FIG. 1 illustrates in cross section a nacelle 3 on an aircraft (not shown) which contains a gas turbine engine (not shown) which powers a fan 6. the temperature of the inlet airstream 9 must be measured in order to control the operation of the engine. However, a temperature sensor positioned as shown by sensor 12 may become damaged by incoming debris, such as dirt, hail, snow, and birds. Further, a sensor such as sensor 12 will be affected by the anti-icing apparatus which heats the nacelle 3: the sensor 12 will not register the actual temperature of the incoming airstream.
One approach to preotecting the sensor 12 locates it within a protective duct 15 in FIG. 2. However, the temperature of the duct 15 itself will affect the temperature of the air flowing through it, with the result that sensor 12 located within the duct 15 will have a time lag in response to changes in temperature of the air passing through the duct. Such a time lag is not desirable.
The ejector 18 is connected by a duct 21 to a source of high pressure air, such as compressor air bled from the engine (not shown). The ejector ejects an airstream 24 into the duct which draws upstream air in region 27 across the temperature sensor. The ejector 18 also functions to reduce the effect of the duct temperature on the airstream striking the sensor. One problem which results from the use of ejectors is that the inlet air velocity is greater at cruise than at takeoff. If the ejector 18 is designed to draw the proper amount of air across the sensor 12 during takeoff, then at the higher speed cruise conditions, an unnecessarily large amount of air is ejected as airstream 24, thus wasting energy.